Two ranchers walk across parched dried soil in Culberson County, Texas. The period between Nov. 1, 2010, and Oct. 31, 2011, was the driest one-year period in Texas history, and June through August was the hottest three-month period ever reported by any U.S. state, according to climate experts. Credit: USDA

For three months last summer, temperatures in Texas soared higher than at any time in recorded history, and the state is still coping with the most expensive drought in its history. But can the 2011 Texas heat wave be attributed to global warming?

Most scientists are careful not to link specific weather events to climate change trends, but NASA's James Hansen and two colleagues from the NASA Goddard Institute for Space Studies and Columbia University have taken that plunge. They've gathered data they say shows that the 2011 Texas and Oklahoma heat wave—as well as a deadly Moscow heat in 2010—were "a consequence of global warming because their likelihood was negligible prior to the recent rapid global warming."

Their conclusions are based on more than 50 years of temperature data, Hansen told InsideClimate News. By comparing the recent shift toward extreme high summer temperatures with that data, he said his group was able to demonstrate that the record-breaking 2011 Texas heat wave wouldn't have occurred without global warming. This data also provides a broader context for the summer of 2011, which across the United States was the second warmest on record, with the National Oceanic and Atmospheric Administration's Climate Extremes Index twice the historical average.

Making a connection between the Texas heat wave and climate change could have profound practical and policy implications because, as Hansen and his colleagues write, the global warming trend "has been attributed with a high degree of confidence to human-made greenhouse gases."

Hansen, who directs the Goddard Institute, has become a target for climate change skeptics who say his activism undermines his science. But his scientific standing is so solid that his research continues to be published in respected scientific journals, including Environmental Science & Technology, Atmospheric Chemistry & Physics and the Review of Geophysics.

"Our paper deals with the frequency of hot seasons, mainly hot summers, because that is the most important season," Hansen said. "The times and places with hot summers tend to be where the weather is dominated ... by high pressure, so there is high correspondence between the [extremely hot outliers or extreme] heat waves and drought conditions." They also point out that summer, "when most biological productivity occurs, is the most important season for humanity and thus the season when climate change may have its biggest impact."

Other climate scientists say that while the data Hansen and his colleagues have gathered is solid, they're not yet ready to draw such a definitive conclusion.

"The paper makes key arguments, but we haven't yet done the definitive analysis." said Don Wuebbles, professor of Atmospheric Sciences and Electrical and Computer Engineering at the University of Illinois. Wuebbles has also been a research scientist at Lawrence Livermore National Laboratory, and he shared the 2007 Nobel Peace Prize for his work with the Intergovernmental Panel on Climate Change (IPCC).

"It used to be that when an event like that [the Texas heat wave] would happen, a scientist would say 'We can’t say if that change is because of climate change,'" Wuebbles said. "But because of new analysis that looks at the probability of such an event occurring, we can say the likelihood of such an event occurring is greatly increased because of climate change." What's new in the Hansen paper, Wuebbles said, is "tying a specific event to climate change patterns."

Martin Hoerling, a research meteorologist with NOAA's Earth System Research Laboratory, thinks it's still too early to draw firm conclusions about the cause of the Texas heat wave. He also warned that the high Texas temperatures can't automatically be linked with the drought.

"The planet as a whole is warming as a result of greenhouse gases. That science is very solid," he said. "But we don't fully understand the variation from place to place."

Although the year that ended on September 30, 2011 was the driest recorded in Texas since 1985, Hoerling said the extreme absence of moisture resulted from a La Niña cycle event that kept summer tropical storm precipitation from falling on Texas. He acknowledged that the drought has been exacerbated by the extreme summer temperatures. But he said the heat wave "could not have been anticipated" despite the global warming trends. He explained that while the planet has warmed 1 degree Celsius over the past century, Texas has not had a comparable temperature increase, so what happened in 2011 couldn’t have been predicted. "What happened in Texas is uniquely related to variability. Not all places are warming at the same rate," said Hoerling.

Hansen pointed out that his group's study focused only on temperature data, because at this point there isn't enough data to do a comparable analysis of precipitation. But given what is known, he said "there is every reason to believe that the trend toward greater variability, larger anomalies, is true for precipitation as well as temperature."

Temperature greatly influences how much water vapor air can hold, he explained, and "there are multiple indications that moisture falls in more extreme events as the planet warms.

"We can expect that 100-year floods and 500-year floods will be occurring much more frequently than they did in earlier climatology," he said, but "that is not what our paper deals with."

What the paper does focus on, Hansen said, is determining whether extreme weather events like the Texas heat wave can be attributed to climate variability—the natural ups and downs in seasonal temperature—or to the global upward trend in summer temperatures that science now links with climate change.

How this paper will be received remains to be seen. A spokesperson for Senator James Inhofe (R-OK), a leading Congressional climate change skeptic, is already discounting the study because of Hansen’s participation.

"Hansen has lost a lot of credibility ever since he's moved to be more of an activist than a scientist," said Matt Dempsey, communications director for the Senate Environment and Public Works Committee Minority office, which Inhofe leads. "The American people have rejected alarmism on global warming. Global warming has all but gone away on Capitol Hill."

Dempsey also dismissed the IPCC's conclusion, drawn from the work of hundreds of scientists worldwide, that global warming is unequivocal and can be attributed to the increase in greenhouse gasses that are rising due to human activity.

This new paper will likely do little to reconcile the gap between climate scientists and policymakers like Inhofe. But as the political debate continues, government agencies and departments from the U.S. Geological Survey to the Department of Defense are using scientifically observed temperature and climate trends to engage in short- and long-term planning.

At the same time many scientists, including Wuebbles, note that what they've been observing in the way of global warming is now matching what scientific models of climate change have predicted. But to fully understand what's actually happening to the climate—and whether specific events are tied to global changes—we need better ways of analyzing extreme weather events, he said.

Regardless of politics and whether or not it was a result of climate change, Washington, D.C. experienced record-breaking high summer temperatures last year. In fact, July was the capital's hottest month since record-keeping began in 1871.